1. Field of the Invention
The present invention relates to a temperature compensating dielectric ceramic composition and, more particularly, a dielectric ceramic composition for temperature compensating monolithic ceramic capacitors.
2. Description of the Prior Art
So far, dielectric ceramic compositions of a system MgTiO.sub.3 -CaTiO.sub.3 have been used as a dielectric material for temperature compensating monolithic ceramic capacitors.
Such monolithic ceramic capacitors are generally manufactured by a process comprising the steps of preparing ceramic green sheets, printing conductive metal paste on each green sheet to form internal electrodes, stacking the printed green sheets one on another, press-fitting the resultant stacked sheets to form a monolithic multi-layer body, and then firing the same in air at a high temperature of not less than 1300.degree. C.
Thus, when manufacturing the monolithic ceramic capacitors with the above dielectric ceramic composition, it is required to use a metal, which does not melt at the sintering temperature of not less than 1300.degree. C. and does not oxidize even if heated to that temperature in air, as a material for the internal electrodes. Such a requirement is fully met by noble metals with a high-melting temperature such as Pd, Pt, etc. However, these materials are quite expensive, so that the use of noble metals as a material for internal electrodes has resulted in increase of manufacturing costs of the monolithic ceramic capacitors. In addition, firing at the above high temperature results in increase of the cost of firing.
In order to solve such problems, attempts have been made to use inexpensive base metals as a material for internal electrodes. The base metals are oxidizable and reactable with the dielectric material in the oxidizing atmospheres, so that it is required to fire the dielectric material with electrodes in reducing or neutral atmospheres. However, the compositions of the system MgTiO.sub.3 --CaTiO.sub.3 are reduced greatly in such nonoxidizing atmospheres, resulting in considerable lowering of the insulation resistance because of reduction of the oxides.
To this end, some of the inventors have proposed in U.S. Pat. No. 4,816,429 to use, as a dielectric material for temperature compensating monolithic capacitors, a dielectric ceramic composition consisting essentially of barium oxide silicon oxide and zirconium oxide, or a dielectric ceramic composition consisting essentially of barium oxide, strontium oxide, silicon oxide and zirconium oxide, said three or four components, when calculated in terms of BaO, SrO, SiO.sub.2 and ZrO.sub.2 respectively and expressed by the general formula: EQU x(BaO.sub.1-.alpha. SrO.sub..alpha.)-ySiO.sub.2 -zZrO.sub.2
(wherein x, y and z are weight percentages of respective components and x+y+z=100, and 0.ltoreq..alpha..ltoreq.0.90), having compositional proportions falling within the polygonal area defined by the points A, B, C and D in a triangle phase diagram, the sets of x, y and z at said points A, B, C and D being as follows:
______________________________________ x y z ______________________________________ A 50 49 1 B 50 20 30 C 15 20 65 D 15 84 1 ______________________________________
The above composition has a low sintering temperature of not more than 1000.degree. C. and possesses a high specific resistance of 10.sup.12 .OMEGA.-cm even if fired in nonoxidizing atmospheres. Thus, the composition makes it possible to produce monolithic ceramic capacitors with internal electrodes of a base metal.
However, it has now been found that such a dielectric composition possesses a large temperature coefficient of capacitance at the side of low temperature. For example, its temperature coefficient of capacitance at -55.degree. C. exceeds .+-.100 ppm/.degree. C. even if the composition has the temperature coefficient of capacitance at +125.degree. C. is less than .+-.100 ppm/.degree. C. In addition, the temperature coefficient of capacitance varies nonlinearly with temperature. If such a dielectric ceramic composition is applied to a temperature compensating capacitor to use it in a radio frequency (RF) tuning circuit comprising a combination of a coil and a capacitor, the output RF signal taken from the tuning circuit varies with temperature. Thus, it is difficult with the above dielectric ceramic compositions to produce temperature compensating capacitors for RF tuning circuits as they are required to have a small temperature coefficient of capacitance, linear temperature characteristics and a high Q factor.